The production of hydrocarbons from an oil or gas well is often accomplished with an electrical submersible pump (ESP) system that includes multiple tools, such as one or more motors, pumps, gas separators, etc., in a linear, generally tubular configuration forming a tool string suitable for being lowered into a wellbore. The various tools are fixedly connected to each other at their ends such that, when the tool string is suspended vertically in the wellbore, each tool supports the weight of the tools therebelow. A shaft runs through the tool string along a longitudinal axis; this shaft may include separate shaft sections for each of the tools, which may be mechanically coupled together at their ends to transfer rotational motion from one section to the next.
During use of the ESP system to pump hydrocarbon fluids from the bottom of the well to the surface, the pumped fluid generally flows inside the tool string through flow passages contained in an annular region surrounding the shaft. The pumped fluid may be laden with an abrasive such as sand, which tends to cut into the tool housings. Continuous abrasion over a long period of time can ultimately result in the complete breaking of a tool into two parts. When that happens, the lower part of the tool as well as all tools suspended therefrom (hereinafter collectively referred to as the “lost unit”) fall to the bottom of the well. In order to allow continued use of the well, a time-consuming and expensive “fishing job” is then usually undertaken to retrieve the lost unit. The fishing operation can take days or even weeks, and can cost on the order of a hundred thousand dollars. In some instances, the unit is irretrievable, and is shoved to the bottom of the well and abandoned. In the worst case, the well itself may be lost as a result, potentially causing economic damage of millions of dollars.
In the drawings, various shadings and hatchings are used to visually distinguish different components of the depicted tools. These shadings are not intended to indicate the types of materials used for the various components, and are, accordingly, not to be interpreted as limiting the scope of the depicted embodiments.